In present trench type power MOSFET devices, a vertical gate oxide is formed simultaneously within the vertical walls of the trench and at the trench bottom. In order to provide a low RDSON, the vertical oxide should be relatively thin. However, the gate to drain capacitance is determined by the thickness of the gate oxide at the trench bottom, and the gate breakdown voltage VGSMAX is limited by the curvatures of the oxide at the trench bottom corner. Thus, the desire for a thin vertical gate oxide for low RDSON is contradictory to the need of a thick oxide at the bottom of the trench for improved VGSMAX and a low gate to drain capacitance. It would be desirable to harmonize these trade-offs.
A further problem exists in present trench type power MOSFETS due to the conventional formation of the channel and source diffusions. Thus, these regions are usually formed by an implant followed by the diffusion. The implants are known to cause surface damage which extends to a particular depth, depending on the process variables. Therefore, the vertical channel, which extends from the point at which the source intersects the trench wall to the bottom of the channel diffusion, will include damaged silicon caused by the earlier implants. This then increases threshold voltage and increases the channel resistance. It would be desirable to avoid the influence of implant damage on the conduction channel of the device.
A still further problem exists in current power MOSFETs in that the inherent Miller capacitance of the structure increases the gate charge QG of the device and thus increases switching loss. It would be desirable to reduce Miller capacitance to reduce switching loss.